In some applications it is important to know the load on a bolt, e.g. in applications such as pressure boundary bolted joints (piping and pressure vessel gasketed, flanged, bolted joints).
A bolt that is overloaded can cause failure of joint components, whereas a bolt that is under-loaded may be at risk of fatigue failure, may be indicative of adjacent bolts being overloaded and/or may result in leakage between pieces clamped by the bolt. The load on a bolt can change over time as the nut loosens or the bolt or other aspects of the structure creep. This is particularly the case with joints in vibrating services and high temperature joints. Monitoring the residual bolt load during assembly ensures accurate bolt load is applied and monitoring during operation enables proactive retightening of the bolts as required.
Bolt load monitoring methods tend to fall into two categories, contact measurement methods and non-contact methods. Contact methods use a mechanical means of measuring the relative displacement of two datums, such as a dial gauge or lever-mechanism.
To the inventor's knowledge, all non-contact methods (such as light wave measurement, ultrasonic measurement and capacitive gap measurement) involve measurement tools that (relative to contact tools) are more expensive, more reliant on datum condition to establish a repeatable reading and more susceptible to damage and degradation at elevated temperature.
In the inventor's experience with commercially available bolt load measurement devices, accurate load monitoring requires that each bolt is measured in its unloaded condition to determine an initial relative disposition of the datums. That measurement is retained in a way that it can later be retrieved and correlated with the bolt for subsequent load calculations. In the subsequent calculations, a measured relative disposition of the datums whilst the bolt is under load is compared to the initial relative disposition to determine the relative displacement of the datums associated with the load.
The inventor has recognized that taking an initial measurement, storing and retrieving the information, and performing these calculations is laborious and another potential source of error.
Various contact methods and devices therefor are disclosed in international patent publication no. WO 2010/140002 A1. FIGS. 3, 5 and 4 of that publication are reproduced as FIGS. 1, 2 and 3 herein. To the inventor's knowledge at the time of writing, the device disclosed in FIGS. 2 and 3 does not correspond to any commercially available product. Whilst not well known, this arrangement goes some way to addressing problems related to the location of the nut.
FIG. 1 shows a bolt 1 carrying a pair of nuts 2. The bolt 1 is a stud bolt having a continuous external thread along its entire length.
A strainable portion 3 of the bolt is bracketed by the nuts 2. When the nuts are tightened to compress a structure (not shown, e.g. two mating flanges of a joint), load is applied to the bolt 1 via the threading engagements of the bolts and the nuts. The portions of the bolts underlying the nuts and including the relevant threaded portions constitute load receiving portions of the bolt 1.
The applied load places the bolt in tension, causing it to strain (or more specifically to extend). “Strain” as used herein refers to a change in a dimension expressed as a proportion of that dimension. It does not imply damage or approaching breakage. Within the elastic range of the material, strain is directly relatable to load via the elastic properties of the material.
The bolt 1 includes a bore, running along most of its length, in which a pin 4 is carried. The inner end of the pin is threadingly engaged with the strainable portion 3. This threading engagement is at the root of the pin. The other end of the pin is a free end adjacent the upper (as drawn) end of the bolt.
The free end of the pin and the upper end of the bolt are datums. When the bolt is stretched, the pin (which remains unstressed) recedes into the bolt 1. This recession can be measured with a dial gauge 6 and indicates an amount by which the strainable portion is strained.
The amount of recession depends on the location of the nut. The location of the nut can vary, depending on the thickness of the clamped structure. Calculations to correct for such variations are laborious and a potential source of error.
In the device of FIGS. 2 and 3 only a lower (as drawn) end of the sleeve 8 is anchored in to the bore of the strainable portion so that the top (as drawn) of the pin and the sleeve provide datums from which measurements unaffected by the position of the nut may be taken.
Various non-contact methods and devices therefor are disclosed in UK patent application no. GB 2 372 826 A. Again, to the inventor's knowledge these devices do not correspond to any commercially available products.
FIGS. 2 and 4 of UK patent application no. GB 2 372 826 A are reproduced as FIGS. 4 and 5 herein. A gauge pin 4 within a bolt is disclosed. The end surface 10 of the pin is prepared to reflect light in a manner indicative of axial movement. The pin is potentially integral with the bolt and formed by machining out surrounding material. A fibre optic probe 12 conveys light reflected from the pin to an imaging spectrometer to obtain an indication of the axial movement.
The variant of FIG. 2 of UK patent application no. GB 2 372 826 A (herein FIG. 4) incorporates a recess 14 dimensioned for a close sliding fit with the probe. This recess is said to engage with and align the probe.
The variant of FIG. 4 of UK patent application no. GB 2 372 826 A (herein FIG. 5) shows a bore such that the gauge pin 4 is wholly within the strainable portion of the bolt. The bore is enlarged to define a step 16 adjacent the end 10 of the gauge pin. Appreciable radial clearance between the probe and the bore is illustrated. A compression spring 18 is loaded by a collar to bias the probe against the step to fixedly locate the probe relative to the step.
In this art, a bore such as the bore of FIG. 4 of UK patent application no. GB 2 372 826 A (herein FIG. 5) would typically, if not universally, be formed by drilling. Drilling, when performed carefully by a skilled machinist using high quality tools and equipment, can be expected to produce a bore having an H10 tolerance at best, that is a bore the widest and narrowest locations of which fall within the H10 range. In practice, the tolerance would likely be wider than H10 because there is no apparent reason to incur the expense of maintaining a narrow tolerance.
To the inventor's knowledge, at the time of writing the use of contactless machining to form load indicating fasteners was not publicly known. Contactless machining is an umbrella term taking in Electro-Discharge Machining (EDM), Electro-Chemical Machining (ECM) and laser machining.
Sinker EDM entails a shaped electrode (tool) being held at a high voltage relative to a workpiece and slowly moved towards the workpiece so that an arc is formed between the two. This arc has the effect of eroding the workpiece into a shape complementary to the shape of the electrode. Typically the electrode and relevant portions of the workpiece are bathed in a dielectric fluid and arcs form when the dielectric fluid breaks down.
U.S. Pat. No. 4,847,464 discloses the use of sinker EDM to form a spinneret capillary. International patent publication number WO 2012/097187 discloses EDM disintegration to remove residual tap, bolt or rivet materials. U.S. Pat. No. 5,391,850 discloses drilling small diameter holes with high accuracy using fast-hole EDM.
Fast-hole EDM entails rotating and advancing a tubular electrode through which dielectric fluid is pumped. Fast-hole EDM can be used to create through holes and blind holes. To create a through hole, an electrode having a simple annular cross-section may be used. To create blind holes, more elaborate electrodes are used to ensure material at and about the axis of rotation is removed. Examples of such electrodes are described in U.S. Pat. No. 3,622,734 A and European patent publication number EP0634243.
The inventor's investigations have entailed confidentially contacting numerous persons skilled in the art of EDM, including many experienced machinists. This investigation has revealed that EDM and other contactless methods are generally considered unsuitable for forming formations similar to the gauge pin of UK patent application no. GB 2 372 826. Indeed the vast majority of the machinists contacted were confident that EDM was not feasible for this purpose:
Most considered sinker EDM technology to be much too slow (i.e. expensive), e.g. forming a 25 mm long pin by this method is generally considered to take more than 2 hours.
EDM disintegration is generally considered to be inaccurate, so much so that if applied to machining out surrounding material to form a gauge pin, it is doubtful that any pin would be left at all.
Likewise fast-hole EDM is generally considered to be unsuitable for leaving an intact central pin. When using a “through hole” electrode, the material from at and about the axis of rotation is typically a misshapen unwanted by-product tapered by preferential erosion at its leading end. Typical “blind hole” methods destroy the material at and about the axis of rotation.
Forming the step 16 and end 10 (of the gauge pin 4) of UK patent application no. GB 2 372 826 A (herein FIG. 5) with any degree of accuracy presents some challenges. The present inventor considers that it would not be commercially feasible to do so using conventional methods. Machining at the bottom of a relatively long slender hole is required. This is problematic for most conventional methods. It means long slender tools, which means higher speed, very small cuts and lots of small movements required. Micro-milling is one option, but it is considered too slow (i.e. expensive) to be commercially feasible.
It is not admitted that any of the information in this patent specification is common general knowledge, or that the person skilled in the art could be reasonably expected to ascertain or understand it, regard it as relevant or combine it in any way at the priority date.